Many unions in Norway in the private sector (if not most) generate statistics based on information from their members regarding salary, education, experience and so on, so for me as a developer/sysop there really is no need to check what my colleagues are paid. I work at a place where everyone has a different background, so I don't expect the same pay for everyone. But I expect that I'm paid similar to others with the same responsibility, tasks, background, etc within the industry and geographical area.
It's just a guess that there is a connection between the difference of perceived wealth and actual wealth of a countries citizens and the secrecy around salaries.
If you earn right the median income , but think the actual median is much higher, you tend to not want to talk about your income because you are of low status, compared to everyone else who you expect to earn more.
Also Norwegians probably do not have to look up their colleagues salaries, if they do not want to.
I would definitely like to know at least the general distribution.